The invention relates to lithography, and in particular, to optical and process correction techniques for lithography masks.
The fabrication of integrated circuits on a semiconductor substrate typically includes multiple photolithography steps. A photolithography process begins by applying a thin layer of a phltoresist material to the substrate surface. The photoresist is then exposed through a photolithography exposure tool to a radiation source that changes the solubility of the photoresist at areas exposed to the radiation. The photolithography exposure tool typically includes transparent regions that do not interact with the exposing radiation and a patterned material or materials that do interact with the exposing radiation, either to block it or to shift its phase.
Areas of the photoresist that are not exposed to the radiation do not change in solubility, so those unexposed areas (if xe2x80x9cnegative photoresistxe2x80x9d is used), or the exposed areas (if xe2x80x9cpositive photoresistxe2x80x9d is used) can be washed away by a developer, leaving patterned photoresist on the substrate. The pattern on the photolithography exposure tool is transferred or printed onto the photo resist. The patterned photoresist is then used as a protective layer during a subsequent fabrication step, such as etching an underlying layer or diffusing atoms into unmasked areas of the substrate.
xe2x80x9cMasksxe2x80x9d and xe2x80x9creticlesxe2x80x9d are types of lithography exposure tools, that is, tools that alter radiation to print an image on the exposed surface. The term xe2x80x9cmaskxe2x80x9d is sometimes reserved for photolithography exposure tools that print an entire wafer in one exposure, and the term xe2x80x9creticlexe2x80x9d is sometimes reserved for a photolithography exposure tool that projects a demagnified image and prints less than the entire wafer during each exposure. The term xe2x80x9cmaskxe2x80x9d is more commonly used generically, however, to refer to any type of lithography exposure tool. The term xe2x80x9cmaskxe2x80x9d is used herein in its broadest sense to mean any type of lithography exposure tool, regardless of the magnification, the type of exposing radiation, the fraction of the wafer that is printed in each exposure, or the method, such as reflection, refraction, or absorption, used to alter the incoming radiation.
A photolithography mask typically comprises a quartz substrate with a patterned layer of opaque chromium that corresponds to the circuit pattern to be transferred to the substrate. A mask can also include a material, such as silicon nitride, that shifts the phase of the exposing radiation. A reduced image of the mask is typically projected onto the substrate, the image being stepped across the substrate in overlapping steps to repeat the pattern.
As each successive generation of integrated circuits crowd more circuit elements onto the semiconductor substrate, it is necessary to reduce the size of the features, that is, the lines and spaces that make up the circuit elements. The minimum feature size that can be accurately produced on a substrate is limited by the ability of the fabrication process to form an undistorted optical image of the mask pattern onto the substrate, the chemical and physical interaction of the photoresist with the developer, and the uniformity of the subsequent process, e.g., etching or diffusion, that uses the patterned photoresist.
When a photolithography system attempts to print circuit elements having sizes near the wavelength of the exposing radiation, the shape of printed circuit elements becomes significantly different from the pattern on the mask. For example, line-widths of circuit elements vary depending on the proximity of other lines. The inconsistent line widths can then cause circuit components that should be identical to operate at different speeds, thereby creating problems with the overall operation of the integrated circuit. As another example, lines tend to shorten, that is, the line ends xe2x80x9cpull back.xe2x80x9d The small amount of shortening becomes more significant as the lines themselves are made smaller. Pulling back of the line ends can cause connections to be missed or to be weakened and prone to failure.
Because of the wave nature of light, even a perfectly straight, opaque edge will not produce a shadow that is absolutely dark in the shadowed areas. A phenomenon known as diffraction causes the light to bend around an edge to produce a pattern of alternating light and dark areas. The width of the alternating areas is on the order of the wavelength of the exposing light and the diffraction pattern intensity falls off rapidly in the shadowed zone. When integrated circuits were fabricated using conductor widths greater than one micron, the effect of diffraction was small and the differences between the pattern on the mask and the pattern produced on the substrate could be ignored. In modem circuits, with conductors widths well under a micron and even under two tenths of a micron, diffraction and other optical phenomena produce effects that are significant in relation to the size of features being produced by photolithography, and such effects can no longer be ignored.
Because the size of the diffraction effects is related to the wavelength of light used, one way to reduce diffraction effects is to use light of a shorter wavelength. The wavelengths used in new photolithography systems have decreased over the years from visible light to ultraviolet to deep ultraviolet. Systems using extreme ultra-violet or soft x-rays are currently being developed. It is desirable, however, to improve the resolution of existing photolithography systems because of the high cost of new systems and because it takes many years for a new generation of photolithography systems to become stable production tools. Moreover, the rate at which shorter wavelength systems are being developed is expected to be insufficient to keep up with the expected reduction in circuit feature size. Thus, it will likely be necessary to overcome diffraction effects, regardless of the wavelength used.
Because it can be determined in many instances how the pattern projected onto the substrate will vary from the mask pattern, the mask pattern can be altered to pre-compensate for the distortion. The printed pattern, rather than the mask pattern itself, then portrays the desired circuit. Techniques for pre-compensating the mask are examples of resolution-enhancing corrections or resolution enhancement techniques. A mask is typically altered by moving features of the mask or adding xe2x80x9csub-resolutionxe2x80x9d assist features, that is, features that are too small to be imaged individually on the substrate, but that scatter or bend light to alter the image of other, larger features on the mask. These xe2x80x9cpredistortionsxe2x80x9d cancel the distortions inherent in the lithography process, resulting in a layout that has improved fidelity to the intended design, improved manufacturing yield, and better circuit performance.
For example, it is known that diffraction effects tend to round off square comers and shorten lines. FIG. 1A shows a pattern 100 on a portion of a mask 102, and FIG. 1B shows the pattern 104 printed by mask 102 onto a substrate 106. Printed pattern 104 is shorter than mask pattern 100 and printed pattern 104 has rounded comers. FIG. 1C shows a modified mask pattern 108 having xe2x80x9cserifsxe2x80x9d 110 added. FIG. 1D shows the pattern 116 projected onto a substrate 118 by mask pattern 108 using serifs 110. Printed pattern 116 is not as shortened as pattern 104 in FIG. 1B and the corners are not as rounded. The use of serifs was described as early as 1981 by B.E.A. Saleh and S. Sayegh in xe2x80x9cReduction of Error of Microphotographic Reproductions by Optimal Correction of Original Masks,xe2x80x9d Opt. Eng., vol. 20, p. 781, and is described more recently, for example, in U.S. Pat. No 5,707,765 to Chen for xe2x80x9cPhotolithography Mask Using Serifs and Method Thereof.xe2x80x9d
It is also known that the diffraction patterns of closely spaced mask pattern features interact. For example, FIG. 2A shows a group of closely spaced parallel lines 202 and an isolated line 206. Isolated line 206 will print a line having a width different from that of lines printed by closely spaced lines 202. Non-uniform thickness in printed lines can interfere with circuit device functioning as described above. Isolated lines can be made to print like the closely spaced lines by adding xe2x80x9cscattering barsxe2x80x9d 210 (FIG. 2B), that is, additional lines on the mask on opposite sides of the isolated line. Scattering bars are described, for example, in U.S. Pat. No 5,821,014 to Chen et al. for xe2x80x9cOptical Proximity Correction Method for Intermediate Pitch Features Using Sub-Resolution Scattering Bars on a Mask.xe2x80x9d Note that a scattering bar 210 is also used along the outside edge of the line 202A, the last line in the closely spaced group. Whether or not a scattering bar is necessary along a particular edge of a feature depends upon the distance to the closest facing feature edge.
Scattering bars, like the serifs described above, are sufficiently thin that they are below the resolution limit of the lithography system and therefore do not appear or xe2x80x9cprintxe2x80x9d on the photoresist. These features do, however, affect the printed image of the nearby features and can make the printed image of the isolated or outside lines, such as line 202A, consistent with the images of the closely spaced lines. Single or multiple scattering bars can be used on both sides of an isolated line.
FIG. 2C shows a portion of a mask 218 having another type of assist feature, anti-scattering bars 220. Anti-scattering bars 220 are actually transparent lines created in an otherwise opaque region 222 of the mask
FIG. 2D shows a portion of a mask 230 having another type of mask correction. The edges 232 and 234 of features 236 and 238 are displaced to widen the features where there is a gap in the feature 240 opposite to the edge. The technique of moving an edge to widen or make thinner a feature is known as xe2x80x9cedge biasing.xe2x80x9d
Many types of assist features have been developed. The following is an exemplary, but not exhaustive, list of design structures that can benefit from the addition of assist features: line ends, line comers, isolated lines, isolated lines adjacent to a set of dense lines, cross or xe2x80x9cXxe2x80x9d structures, and transistor gates.
During the design of masks for fabricating integrated circuits, resolution enhancement techniques are often implemented automatically by automated design tool. The automated design tools review the mask design to locate mask elements that would benefit from a resolution enhancement technique and automatically apply the appropriate technique to the mask. There are two basic strategies used to determine the need to apply a resolution enhancement technique. The first strategy is rule based, and compares the mask features with a set of rules. The second strategy is model-based.
In a rule-based system, the design is checked against a list of rules, and a resolution enhancement technique is applied when a rule indicates that one is required. Rule-based systems are simpler and use considerably less computing resources than do model-based systems. Because of the large number of possible combination of mask feature patterns and the relatively small number of rules, the rules do not correct the mask equally well in all situations, and correction techniques may be added where they are not necessary, thereby unnecessarily increasing the mask complexity.
In a model-based strategy, the image that would be produced by a mask pattern is determined using a software model of the mask, and resolution enhancement techniques are applied only where the model shows them to be needed. Model based strategies are more effective, but require more computational resources.
Although a mask design is ultimately a collection of polygons, the polygons represent circuit features, such as capacitors, transistors, and conductors. On a typical integrated circuit, groups of circuit features are repeated multiple times throughout the design. Circuit designers simplify the design process by treating groups of features as standard cells and reusing them throughout the design. A mask design can, therefore, be analyzed at many different levels of the design hierarchy, for example, from a collection of complex cells, to a collection of individual circuit elements, to a collection of individual polygons. Resolution enhancement techniques can be applied at any level of the design hierarchy. It is typically more efficient to apply the techniques at higher hierarchical levels, so that the techniques need be applied only once for each type of cell. When resolution enhancement techniques are applied to standard cells, however, it may still be necessary to consider how features at the edges of the cells interact with nearby features outside the cell. When a mask is being fabricated, the design is ultimately flattened to a collection of polygons.
One method used by automated software to determine mask corrections entails classifying each edge in the design according to that edge""s proximity to other edges. The distance between an edge and its opposing edge will be referred to as the xe2x80x9cproximity valuexe2x80x9d of the edge. Classification based on the proximity values of edges is referred to as xe2x80x9cspace classification.xe2x80x9d In some implementations, it is the space between the edges that is classified and the edges that define the space are then classified along with the space.
A space classification system is typically described by a table in which classes are defined by ranges of proximity value, and each class prescribes a mask correction treatment.
The mask correction treatment may include, for example, adding one or more assist features, such as scattering bars or edge movements. The mask correction may also include some combination of adding assist features and edge biasing. The space classification definition table describes the position and width of the scattering bars and the extent and direction of the edge biasing.
The term xe2x80x9cedgexe2x80x9d typically means a collection of points, typically on the side of a polygon, and is not limited to mean the entire side of a polygon. One side of a polygon can be divided into multiple edges for convenience of analysis. Thus, a single polygon side can be considered to be a set of edges of different length, and the composition of the set can change during analysis as edges in the set are combined or divided to make a different set of edges. The point at which one edge is considered to stop and a new edge begins is generally chosen to be the point where the proximity value would put the point into a new classification because the distance between the polygon side and the nearest opposing feature may vary along the length of the polygon side. The side of a polygon can therefore be divided into multiple edges that are classified differently.
Edges that are oriented along one of the axes of the mask are referred to as orthogonal shapes. Edges that are oriented at non-normal angles to the axes are referred to as angled edges. In FIG. 3, which will be discussed below in more detail, edge 302, 304, 306, 308, and 310 are orthogonal edges. Edge 316 and 320 are angled edges. Spaces between angled edges and orthogonal edges, such as the spaces designated by letters xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d, and xe2x80x9cDxe2x80x9d of FIG. 3, are handled differently by different design tools.
Some design tools measure the proximity between an orthogonal edge and an angled edge along a line normal to the orthogonal edge. Thus, classifications for a pair of opposing edges, one angled and one orthogonal, are determined based on the orthogonal edge and then applied to both edges. For example, an edge portion of the orthogonal edge 304, such as edge 304B, and its projection 316B on angled edge 316 are thus xe2x80x9cpairsxe2x80x9d and are classified in the same space classification. Edges 316B and 304B therefore receive the same mask correction treatment. In FIG. 3 the treatment shown is a single scattering bar of medium thickness positioned at a prescribed distance from each of the edges.
A coarse classification system is one having few classes and each class is, therefore defined by a relatively broad range of proximity values. The resulting mask corrections, which are typically optimized for edges that have proximity values in the centers of the class ranges, are less than optimum for edges having proximity values at the ends of the class ranges. To produce optimum resolution enhancement, it is desirable to use a classification system that classifies edges into many classes, each defined by a narrow proximity range. Each mask correction is then more closely tailored to the needs of the edge to which it is applied. Such a classification is said to have fine granularity. A fine classification system may include 60 or more classes that cover a proximity range of about two microns or less.
In an actual mask design, the arrangement of edges can be very complex. As shown in space xe2x80x9cDxe2x80x9d of FIG. 3, there can be sudden jumps in the distance between opposing edges and edges can be oriented at non-normal angles to each other. When the opposing edges are not parallel, the distance between opposing edges is different for each point on the edges so different portions of the polygon edge may be classified differently.
As the classification changes, the mask correction specified by the class typically changes. Even with a slight change in proximity, the mask correction for the new class may change substantially. The change in the mask correction is not necessarily linear or in direct proportion to the change in proximity. When using a fine classification system, the classification of a polygon edge can change rapidly, producing extremely short and discontinuous scattering bars or edge movements. The lengths of the edges in space xe2x80x9cCxe2x80x9d, for example, are quite short.
There is a lower limit to the size of features that mask manufacturing can reliably produce. Automated mask inspection tools, which are often used as part of the mask manufacturing process to locate unintended opaque xe2x80x9cspecksxe2x80x9d on the mask, may identify such short assist features or edge movements as defects. Extremely short mask corrections generated by a fine classification system may produce mask assist features or edge movements that do not comply with minimum lengths standards for manufacturability and inspection.
FIG. 4 shows an example of a complex mask design in which each edge portion has generated a scattering bar segments using a classification system having fine granularity. The scattering bar segments, rather than being aligned to produce a smooth scattering bar, form a ragged line. This raggedness produces jogs and short segments, many of which cannot be reliably manufactured.
Thus, while it is desirable to use a classification having fine granularity, such a classification can result in mask design that cannot be reliably manufactured. One solution is to eliminate any scattering bars or other mask corrections that are not manufacturable. While this solution produces a manufacturable mask, it eliminates many mask corrections, thereby reducing the ability of the mask design to pre-compensate for process distortions and reducing the overall fidelity of the lithography process. Another solution would entail writing software algorithms to change assist features or edge movements that are non-manufacturable, for example, by combining corrections. Because of the large number of possible combinations of mask corrections, it would require an impractically large number of rules to anticipate and correct all possible combinations of unmanufacturable mask corrections. Thus, a system is needed to maximize the mask correction treatments while maintaining manufacturability of the mask.
An object of the invention is to improve lithography tools to improve the fidelity of the lithography process.
The present invention comprises methods and apparatuses for implementing resolution-enhancing corrections in lithography tools. In accordance with the invention, edges in the mask design are assigned to classes that specify rules for the application of mask correction techniques. The classes are defined by ranges of proximity values. Each edge is classified based upon its proximity to its nearest opposing edge and based on an additional criterion, such as a minimum length that can be reliably manufactured and inspected.
The invention includes not only methods for designing a mask, but also software for implementing the methods, a computer programmed to carry out the methods, a computer implementable description of a mask design determined by application of the methods, the fabrication of a mask designed by the methods, and the mask designed by the methods.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.